Hydrogen sulfide gas treatment of Cr(VI)-contaminated sediment samples from a plating-waste disposal site - Implications for in-situ remediation

Twenty sediment samples were collected at depths ranging from 5 to 100 feet beneath a chromate-contaminated plating-waste sire and analyzed for Cr(VI) , total chromium, and related constituents. Three of the samples were selec ted for treatment with dilute hydrogen sulfide (H2S) gas to evaluate this a pproach as a possible in-situ remediation technique. Gas treatment was perf ormed in soil-packed columns using 100 ppm (mu L L-1) H2S mixtures, and tre atment progress was assessed by monitoring the breakthrough of H2S. Evaluat ion of treatment efficacy included (1) water-leaching of the treated and un treated columns for 10 days, (2) repetitive extraction of treated and untre ated subsamples by water, 0.01 M phosphate (pH 7) or 6 M HCl solutions, and (3) Cr K-edge X-ray absorption near-edge structure (XANES) spectroscopy of treated and untreated subsamples. Results of the water-leaching studies sh owed that the H2S treatment decreased Cr(VI) levels in the column effluent by 90% to nearly 100%. Repetitive extractions by water and phosphate soluti ons echoed these results, and the extraction by HCl released only 35-40% as much Cr in the treated as in the untreated samples. Analysis by XANES spec troscopy showed that a substantial portion of the Cr in the samples remaine d as Cr(VI) after treatment, even though it was not available to the water and phosphate extracting solutions. These results indicate that the residua l Cr(VI) was sequestered in unreacted grain interiors under impermeable coa tings formed during H2S treatment However, this fraction is immobile and th us unavailable to the environment.